The present invention relates to a control device for a position-control of an implement coupled to a tractor by a hoisting unit.
A control device for controlling a position of an implement coupled to a tractor, of the type under consideration, has been known and disclosed, for example Ismail, Singh and Gee-Clough in "A Preliminary Investigation of a Combined Slip and Draught Control for Tractors", I. Agric, Engng, Res. (1981) 26, 293-306. In this known control device, a traction force or draught control system is combined with the slip control system on the tractor provided with a plow hinged thereto. It has been proposed for a slip control to measure a drive speed by a speed sensor on a driven rear wheel or in the drive and to also measure a travelling speed relative to the soil by a speed sensor on the non-driven front wheel of the tractor. This slip control has, however, the disadvantage that it is suitable only for the tractor in which not all the wheels are driven. In the tractors with the drive for all wheels the draught and slip control disclosed in the above mentioned publication is not expedient because an additional fifth wheel for determining an absolute speed is too expensive. It has been also proposed in the above publication that the travelling speed of the tractor relative to the ground be determined by a radar sensor and to thereby define the slip. Such radar sensors are rather expensive and require certain fitting positions in the device.